Among all known surface treatment methods, it is particularly plasma treatment methods, e.g. PVD methods and CVD methods, and particularly plasma-nitrogenation and plasma carbon deposition, which find increased use as modern hardening and coating methods.
For performing a plasma treatment method with good treatment results, it is required that the treatment process be carried out in a defined gas atmosphere. The gas atmosphere used for this purpose is diluted and has a pressure in the range of a high vacuum.
Therefore, known apparatus for plasma treatment comprise a vacuum container designed for receiving the workpieces and connected to a vacuum source for generating a vacuum in the vacuum container.
Basically, known apparatus of the above type can be used also for plasma treatment of porous workpieces. However, it may occur that foreign substances, especially liquid and solid hydrocarbons, become clogged in the pores and cavities of porous workpieces. Contaminated surfaces and pores, in turn, will release gases into the vacuum atmosphere and thus will disturb the atmosphere that has been set. Contaminations which are not volatile, e.g. burn-off residues, cause the additional disadvantage that they passivate the workpiece surface and thus render the subsequent plasma treatment difficult or impossible. Therefore, for subjecting porous workpieces to a plasma treatment in a vacuum container, the workpieces must first undergo a pretreatment wherein foreign substances are removed from the pores and cavities of the porous workpieces.
In a known pretreatment process, the hydrocarbons are first burned off in a defined gas atmosphere in a burn-off furnace and, subsequently, a mechanical removal process is carried out, e.g. by subjecting the surface to an abrasive stream.
As an alternative to a pretreatment in a specially dedicated apparatus, it is known from German Patent specification 42 28 499 C1 to perform both the pretreatment processes and the subsequent coating of the workpiece in a vacuum container. For pretreatment, the workpiece is e.g. cleaned and heated by a low-voltage arc discharge. Thereafter, the workpiece is coated by a vacuum-arc discharge between a target and an anodic electrode. The apparatus used for this purpose further comprises an evacuating connection. Since the evaporated particles have to be removed from the vacuum container via the evacuating connection, e.g. a vacuum pump, a danger exists that the pump is damaged. Damage can be caused to the pump particularly when a large quantity of disturbing foreign substances have to be sucked, or when a high vacuum has to be generated, since a pump designed for this purpose is particularly sensitive.
Further, from U.S. Pat. No. 4,361,472, a sputtering device is known for sputtering an element by use of a plasma source. This device comprises a collector face whereon the sputtered material is collected, e.g. by subjecting the collector face to an anode potential. To be able to use such a collector face in a device for cleaning and subsequent coating of a workpiece, the collector face would have to be arranged as closely as possible to the workpiece so as to prevent the intrusion of foreign substances into the vacuum pump operated during the cleaning process. If, however, a workpiece has to be cleaned on all sides, the use of respective collector faces cannot reliably prevent that foreign substances reach the pump and damage it.
The precleaning of workpieces is of eminent importance for the subsequent plasma treatment. However, due to the complex preparation of the workpiece and the apparatus required therefore, plasma treatment of porous workpieces is presently performed only in exceptional cases.
It is an object of the invention to provide an apparatus which is suited to perform the plasma treatment of porous workpieces by use of simple means and with continuously high quality.